Project Summary The microbial colonization of the infant gut has life-long implications for health. The intestine of breast-fed infants is commonly dominated by Bifidobacterium, particularly by strains that are capable of digesting human milk oligosaccharide (HMO). The first goal of this work is to identify the species of bacteria negatively correlated with Bifidobacterium and to explore whether the dominance of Bifidobacterium over these organisms is related to the efficiency and mechanism used by Bifidobacterium to consume HMO. Next, this proposal will explore if Bifidobacterium are capable of reducing the levels of antibiotic resistance in the microbiome. Aim 1 uses sequencing based techniques to assess the bacterium negatively correlated with the presence of Bifidobacterium in infant stool samples in vivo. We predict that Bifidobacterium will be negatively correlated with both Bacteroides and members of family Enterobacteriaceae. Also in Aim 1, we will use mass spectrometry to measure the microbial metabolites in stool samples to correlate the presence of remaining carbohydrates in the stool with the bacteria present. In Aim 2, we will then isolate Bifidobacterium, Bacteroides, and members of family Enterobacteriaceae and test the way these species interact in the presence of HMO in vitro. Aim 3 explores the ability of Bifidobacterium to reduce levels of antibiotic resistance genes in the microbiome, using sequencing technology to determine if there is a correlation between antibiotic resistance gene levels and the presence of Bifidobacterium. I anticipate finding lower levels of antibiotic resistance in communities dominated by Bifidobacterium. These experiments will enhance our understanding of the developing infant microbiome and work towards identifying potential prophylactic probiotics for use in infants with the potential to reduce levels of dysbiotic community members such as Enterobacteriaceae in the infant microbiome and at the same time reduce levels of antibiotic resistance.